The present invention relates to an optical sensor system such as oximeter, and more particularly to such a system designed for use in a radio frequency (RF) environment or other environment requiring electrical isolation of the patient.
Many applications exist where it is desirable to deliver and/or receive light in diagnosing or treating patients. In many cases the light source and light detecting apparatus can be located close to the patient. For example, in the case of patient monitoring with conventional pulse oximetry, light emitting diodes (LEDs) and a photodetector are located on opposite sides of and proximate to a patient's finger. The LEDs and the photodetector are controlled by electrical wires which extend to a control and display module. The absorption of light by tissue and blood in the finger is detected and analyzed to measure the patient's heart rate and oxygen saturation.
Using conventional pulse oximetry in the operating room or during magnetic resonance imaging (MRI) examination can be difficult to implement, dangerous for the patient and/or ineffective due to environmental interaction with wires attached to the LEDs and photodetector. In these cases and others involving electric fields, such as RF fields, it can be advantageous to couple the light using fiber optic light guides (e.g., optical fiber bundles) to preclude electrical interaction with the RF or other electrical field.
In the prior art, two pulse oximeter systems have been developed using fiber optic sensors. Both of these designs have limitations, and neither effectively solves the problem of providing adequate patient isolation with good instrument performance.
Nonin Medical developed the 8604FO Pulse Oximeter with fiber optic sensors composed of two fibers. One fiber was used to deliver light to the patient; another was used to receive light from the patient. Invivo Research developed the 4500 MRI Pulse Oximeter with sensors using one optical fiber and one electrical wire bundle. The fiber provided isolation from the light source, but not the photodetector. In both of these systems, the optical fiber sensors were connected between the control and display module and the patient. Because the oximeter contained some ferrous material and because it was desirable to locate the module close to the viewing window, optical fibers were 30 feet long for Nonin and 17 feet long for Invivo.
Both oximeter system designs fail to combine adequate performance with the desired patient isolation. The Invivo Oximeter provides oxygen saturation measurements with a specified accuracy for saturation between 70 and 100%. While the performance is good, the sensor still contains wires which can create a safety concern for the patient. The Nonin oximeter provides the desired patient isolation, but was recalled by the FDA in July, 1992 for ". . . inaccurate arterial blood oxygen saturation readings . . . ."
While the typical MRI environment presents dangers both from the magnetic field and the radio frequency (RF) field associated with the equipment when it is turned on, it is primarily the RF field which is of concern since this can result in burning of the patient and inaccuracies in the reading due to interaction with the electrical fields associated with the oximeter. The magnetic field presents a problem mainly in that objects of a metallic nature may unexpectedly respond to the field and move with considerable speed under the field influence.
Accordingly, it is an object of the present invention to provide an optical sensor system for use on a patient in an MRI or other electrically isolated environment.
Another object is to provide such a system which combines adequate patient isolation with good instrument performance.
A further object is to provide such a system which is easy and economical to manufacture, maintain and use.